1. Field of the Invention
The present invention concerns solenoid-type actuators and in particular actuators of the type whose armatures are disposed in fixed-volume sealed chambers.
2. Background Information
Electromagnetically operated valves ordinarily employ solenoid-type actuators. An armature, often referred to as a xe2x80x9cplungerxe2x80x9d in valve-type applications, is so disposed in a guide as to allow it to reciprocate. The plunger includes ferromagnetic material that forms part of the path taken by magnetic flux that results when current flows in a solenoid coil. The magnetic path""s reluctance varies with plunger position. In accordance with well-known magnetic principles, therefore, the flow of solenoid current results in a magnetic force that tends to urge the plunger in one or the other direction.
In an increasingly large number of valve installations, the power employed to drive the solenoid coil comes from batteries. This makes constraints on power dissipation severe in many instances. In the case of battery-powered automatic toilet flushers, for instance, battery life is expected to be three years or more. A great deal of effort has therefore been devoted to minimizing the energy expended in any given valve actuation.
One result of such efforts is the use of an incompressible fluid to fill plunger-isolating chambers. It is desirable in many applications for the plunger to be isolated from the fluid that the solenoid-operated valve controls. A common approach to achieving the result is to enclose the plunger in a chamber whose closure at one end is provided by a flexible diaphragm. The diaphragm acts as the valve member, i.e., the member that is seated in the valve seat to close the valve and that is withdrawn from the valve seat to open it. Typically in response to the force of a bias spring, the plunger moves to an extended position, in which it deforms the diaphragm into the shape that causes it to seal the valve seat. Typically in response to magnetic force resulting from solenoid-current flow, the plunger is withdrawn against the spring force to allow the valve to open.
To enhance energy savings, a permanent magnet is often used to retain the plunger in the position opposite the one in which the bias spring holds it. To allow the valve to assume the latter (typically valve-closed) position, the solenoid is driven in such a direction as to counter the permanent magnet""s magnetic field and thus allow the spring force to close the valve. An actuator that thus requires power only to change state but not to remain in either state is known as a latching actuator.
Independently of whether the sealed-solenoid-chamber actuator is of the latching type, though, further energy savings can be achieved by filling the closed plunger chamber with an incompressible fluid. To appreciate the advantage that an incompressible-fluid-filled chamber affords, consider the valve operation in which a plunger is moving the diaphragm into its seated position in response to a bias spring""s force. The fluid that the valve controls is usually under pressure, and that pressure will prevail over the diaphragm""s outside face. If the plunger chamber, which is on the other side of the diaphragm, is simply filled with, say, air at ambient pressure, the bias spring will need to overcome the force that the controlled fluid""s pressure exerts. If the plunger chamber is filled with an incompressible fluid such as water, on the other hand, the controlled fluid""s pressure is transmitted to the incompressible fluid within the plunger chamber, and the force that it exerts on the diaphragm""s outside face is canceled by the resultant force on its inside face. The spring therefore does not need to exert as much force as it otherwise would, and this means that the power expended in retracting the plunger against that spring is similarly less.
Thus combining the incompressible-fluid-filled plunger chamber with other energy-saving actuator features has lead to great economies in automatic-valve-actuation use. This is particularly true when the pressure of the fluid being controlled is what actuates the main valve, and the solenoid-operated actuator controls only a pilot valve used to control pressure relief. In such an arrangement, the actuator can be made quite small because the pilot valve it operates is required to control only a relatively small fluid flow.
But we have recognized that this very smallness can detract from actuator longevity when the actuator employs an incompressible-fluid-filled plunger chamber. We have also found a solution to this problem, though. For actuator in which the volume of the incompressible fluid bears a ratio of less than 0.2 cm to the surface area of the plunger-chamber wall, we select the materials of the plunger-chamber wall an incompressible fluid such that the loss of incompressible fluid through the plunger-chamber wall is less than 2% per year. It turns out that a significant factor detracting from the longevity of small actuators employing incompressible-fluid-filled plunger chambers is the loss of the incompressible fluid as a result of diffusion. If the actuator materials are so chosen as to keep the diffusion low, longevity is improved.
The particular combination of materials is not critical so long as it meets the above-mentioned diffusion criterion, but an example combination meeting this criterion is given below.